Aluminum die castings have been used for equipment housings for many years. The many advantageous mechanical properties of aluminum die castings have motivated widespread usage of such die castings for pump housings. In the past, it has been found necessary to use a separate pressure vessel, preferably steel, within the housing to serve as a cylinder for the piston of such pumps due to the perceived poor wear characteristics and porosity of aluminum.
In addition, other parts and subassemblies were often present with the consequent increase in cost and complexity.
In one aspect of the present invention, it has been found possible to eliminate separate parts such as the pressure vessel in such assemblies while at the same time achieving improved wear characteristics over conventional or "soft" aluminum, such as, for example type 380.
The present invention accomplishes this by utilizing the relatively hard but thin surface region or skin of approximately 0.015 to 0.030 depth which appertains to aluminum die casting. In the past, such a skin was typically machined away leaving only the soft (and possibly porous) bulk aluminum which is unacceptable as a working surface. In the present invention, the workpiece is die cast to near net shape and subsequently only finish machined such that a portion of the hard surface region remains in the aluminum and is available as a hard working surface having a desired configuration and surface finish. An additional or side benefit of this invention is that, by leaving a portion of the hard skin on the die casting, the characteristic lower porosity of the skin can be utilized as well.
It is believed that the mechanical properties of die castings are directly related to solidification rate. The aluminum alloy composition can be used to vary the hardness and machinability; the cooling rate and injection pressure can also be controlled to regulate surface hardness of the die casting; entrapped gases can also affect hardness. The microstructural features that affect hardness in such die castings are: i) grain size and shape, ii) dendrite-arm spacing, and iii) the size and distribution of second-phase particles and inclusions.
Another aspect of the present invention is the utilization of portions of the equipment or pump housing to replace discrete parts from prior art assemblies. More specifically, in the practice of the present invention, a pressure control valve housing, fluid return path, and stop for the pressure control valve are preferably cast integral with the pump housing. In addition, a valve seat may be formed integral with the pump housing by machining and coining a conical surface in the housing.